You can't perform on your best level just by training. He did a decent job whenever he had to play and that's all you can ask from a reserve goalkeeper.
Sorry but it's not really like that.
The notion stands for all players,
except the goalkeepers.
The goalkeepers train differently and react differently to sharp situations. If a GK performs much worse compared to his training performances, than he is nervous but his actual level is the same.
There are dozens of special test-exercises a GK has to accomplish and the coach always knows his "level", regardless how often he is getting chances.
Back in my days the "reserve" goalkeepers often played for the B team, just to be current in live situations, and then sat on the bench for the A team, that was a better setup for goalkeepers...